The present invention relates to an exposure apparatus, and more particularly to an exposure apparatus used to manufacture devices, such as a single crystal substrate for a semiconductor wafer and a glass plate for a liquid crystal display (LCD). The present invention is suitable, for example, for an exposure apparatus that uses as exposure light ultraviolet (“UV”) light and extreme ultraviolet (“EUV”) light.
There has conventionally been employed a reduction projection exposure apparatus that uses a projection optical system to project a circuit pattern on a mask (reticle) onto a wafer, etc. to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in the photolithography technology.
The minimum critical dimension to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better the resolution is. Recent demands for finer semiconductor devices have promoted a shorter wavelength of ultraviolet light from an ultra-high pressure mercury lamp (i-line with a wavelength of approximately 365 nm) to KrF excimer laser (with a wavelength of approximately 248 nm) and ArF excimer laser (with a wavelength of approximately 193 nm).
The lithography using the ultraviolet light has the limit to satisfy the imminent demands for finer semiconductor devices. Accordingly, a reduction projection exposure apparatus (referred to as an “EUV exposure apparatus” hereinafter) has been developed which uses EUV light with a wavelength of 10 to 15 nm shorter than that of the ultraviolet light to efficiently transfer a very fine circuit pattern of 0.1 μm or less.
As the exposure light has a shorter wavelength, its absorption remarkably increases in a material, and becomes incompatible with use a refraction element or lens for visible light and ultraviolet light. No glass material is compatible with a EUV light's wavelength, and a reflection-type or catadioptric optical system is used which utilizes only a reflective element or multilayer mirror. A reticule also uses a cataoptric reticle that uses an absorber on a mirror to form a pattern to be transferred.
The mirror cannot reflect all exposure light, but absorbs 30% or greater of the exposure light. The energy of most of the absorbed exposure light turns to residual heat and causes temperature rises of mirror and the reticle. The temperature rises even in a mirror holder that supports the mirror and a reticle chuck that absorbs the reticle due to the residual heat.
At the exposure time, the exposure light is guided to an illumination optical system, a reticle, a projection optical system, and a wafer in this order, and each component generates the heat as a result of absorption of the exposure light. In other words, the temperature rises in mirrors in the illumination and projection optical systems, in the reticle, and in the mirror holders and the reticle chuck due to heat transmissions from the mirrors and reticle. However, the temperature rise gradually mitigates through repetitive exposures and becomes almost constant at a steady temperature.
The EUV exposure apparatus is used for exposure of a circuit pattern of 0.1 μm or smaller, and requires mirrors (in particular, a reflective surface of a projection optical system's mirror) and a reticle to have highly precise surface shapes. A shape error budget a (rms value) is given in the Marechal's criterion as Equation 1 below, where λ is a wavelength of EUV light, and n is the number of mirrors in the projection optical system:
                    σ        =                  λ                      28            ×                          n                                                          (        1        )            
For example, where the EVU light has a wavelength of 13 nm and the projection optical system uses four mirrors, the shape error budget a becomes 0.23 nm. When the temperature rises in mirrors in the projection optical system as a result of exposure light absorptions, a deformed surface shape possibly exceeds the permissible shape error, and makes imaging performance insufficient, i.e., lowered resolution and contrast and insufficient transfers of fine patterns.
Accordingly, these mirrors and reticle use a low thermal expansion glass having a low coefficient of linear expansion, e.g., 10 ppb for reduced heat deformations of their shapes due to the temperature changes. Since the low thermal expansion glass has low rigidity and it is generally made thick to reduce deformations by external forces. This requires, however, an extremely long time for a temperature distribution to turn to a steady state.
The thermal strain amount varies while the temperature distribution turns greatly (to the steady state), and the mirrors and reticle vary their surface shapes, positions, etc., making the transfer accuracy too low for exposure, and reducing the throughput. In addition, a long standby time to exposure or long non-exposure time would decrease the temperatures of the mirrors and reticle, which have been at the steady state, and the exposure restart delays by several hours or longer for thermal stability.
Accordingly, there has been provided an exposure apparatus that provides means for heating a mirror other than the exposure light, makes constant the heat absorbed by the mirror, and maintains the thermal stability when the exposure light is shielded (see, for example, Japanese Patent Publication No. 5-291117).
However, it is difficult for heating means having a wavelength different from that of the exposure light to heat a mirror and reticle like the exposure time, because the mirror and reticle have different absorptive coefficients to optical energy according to light's wavelengths. Strict coincidence of the temperature rise with that at the exposure time requires one by one heating of a mirror, making the structure complicated and increasing the cost.
It is conceivable to repeat the same action between the exposure time and the standby time that waits for exposure. However, the standby time that iterates, similar to the exposure time, a procedure that includes the steps of introducing a wafer, fixing the wafer on the wafer chuck, irradiating the exposure light onto the wafer, and taking out the wafer, causes problems, such as arduous wafer carrying in and out, and worn contact parts in a wafer feeding system.
If a wafer chuck absorbs the wafer like the exposure time at the standby time, the wafer temperature rises and the wafer chuck temperature rises similarly through the wafer. Since the exposure time usually carries in the wafer one after another and carries it out after irradiating the exposure light to it, the wafer temperature does not rise greatly.
On the other hand, if the standby time maintains the wafer absorbed by the wafer chuck like the exposure time, the wafer chuck's temperature becomes higher than that at the actual exposure time. This means that the wafer chuck has different temperature distributions between the exposure time and the non-exposure time. Therefore, when the exposure starts form the standby time, the temperature of the wafer chuck that absorbs the wafer varies, deforms the wafer, and deteriorates the transfer precision.